It is known to use low molecular weight gases or liquids in order to kill Ziegler-Natta catalyst systems to reduce the catalyst system activity to a level where the polymerisation stops. Ziegler-Natta catalyst systems usually employ a titanium chloride transition metal component and an aluminum alkyl co-catalyst component or activator.
EP 116917 (Ruhrchemie) for example describes a killer of CO.sub.2 and alcohols. These products are said to react with catalyst to form non-volatile compounds, not active in polymerization.
The use of water as a Ziegler-Natta catalyst killer is disclosed in U.S. Pat. No. 4,701,489 (El Paso). However whilst water is a known effective catalyst killer, acid may evolve and at high levels corrosion may be caused.
It is also known to use high molecular weight products (polyglycols; epoxides; ethylene copolymers; organic titanium compounds; alkoxysilanes; peroxides; zeolites as a water carrier; or surface-active agents) as Ziegler-Natta catalyst killers.
EP 162274 (Ruhrchemie) discloses a high pressure Ziegler-Natta catalyzed polymerization process involving deactivation with an oxygen-containing waxy ethylene copolymer. The purpose is to prevent polymerization of residual monomer in the separator and gas circulating system. EP 140131 (Ruhrchemie) uses polyglycols for similar purposes.
DE-3322329 (Mitsubishi Petrochemical) discloses killers of peroxides which break up into a complex mixture of volatile CO.sub.2 and other mainly volatile components.
It is further more known from JP-A-57158206 (Sumitomo) to use in a slurry polymerization process a mixture of water, sorbitan alkyl ester, and an aliphatic C.sub.3 -C.sub.8 hydrocarbon as catalyst killer to avoid use of a large amount of water, presumably by using the sorbitan alkyl ester to emulsify the water in the C.sub.3 -C.sub.8 solvent. EP-B-71252 (Sumitomo) discloses use of a suspension of water containing fatty acid salt in a hydrocarbon for a similar purpose. The fatty acid salt may act to neutralise acids formed by the water-catalyst reaction.
In recent years use of metallocene based polymerisation catalyst systems using metallocenes as the transition metal component has been suggested; generally using alumoxane as a cocatalyst. For the purpose of this text the term Ziegler-Natta catalyst systems is used to exclude metallocene/alumoxane systems. The metallocene based systems employ relatively small, molecules of generally unsupported (particularly in high pressure processes) metallocene transition metal components which can have a significant, although still low, vapor pressure at conditions for separation of polymer and unreacted monomer. The cocatalyst has generally a much higher molecular weight than conventional aluminumalkyl cocatalysts and may be an alumoxane as afore mentioned or other suitable cocatalyst complex. However these compounds may still have an appreciable vapor pressure at separation conditions. Often the cocatalyst is used in great excess over the metallocene but the overall catalyst system has a high activity so that catalyst concentrations can be low.
EP-35242 (BASF) uses methanol as a catalyst killer for such metallocene/alumoxane systems; DE 3127133 (Hoechst) uses n-butanol.
Metallocene/alumoxane based catalyst systems have been proposed in which water is introduced into the polymerization zone to create alumoxane in situ (See DE 2608933 Kaminsky; Exxon EP 308177). EP 308177 (Exxon) uses water in the monomer feed to activate, not deactivate, TMA separately introduced as part of the catalyst system.
EP 328348 (Mitsui Petrochemical) uses water in addition to alumoxane and optionally an organoaluminum compound to improve catalysis in low pressure conditions with water (See Example 1 of EP 328348) used to kill catalyst activity. However no elevated pressure is used and there is no recycling because the polymerisation in Example 1 is a batch procedure.
These metallocene based catalyst systems may be used in high pressure polymerisation (See EP 260999 and DE 3150270) including continuous processes involving a recycling and recompression of unreacted monomer.
In industrial-scale high pressure polymerisation a monomer feed is supplied continuously by a compressor installation, polymerised under pressure in a tubular or autoclave reactor, then removed and supplied to a separation stage which may involve high and low pressure separation whereupon polymer is isolated and unreacted monomer is recycled. The extent of recycling can vary depending on the reactivity and amount of comonomers.
The killing of metallocene based catalyst systems in elevated pressure systems with recycled monomer streams may be especially critical. Residual catalyst activity may occur in the separation and in the recycle system; catalyst activity may be highly sensitive to residual killer present in polymerisation. The amount of catalyst system and killer injected need to be appropriately controlled.
It is the object of the invention to provide a commercially viable catalyst killing technology to provide sustained, controllable polymerization with high activity metallocene catalyst systems in fluid systems such as high pressure polymerisation. .It is a further object to maintain the specific advantages of metallocene catalyst systems such as the capability of producing narrow molecular weight distribution (MWD), and/or narrow compositional distribution (CD) of the polymer products and high productivity of the catalyst system.
In particular it is amongst the objects of the invention to provide a catalyst killing technology which provides a quick killing reaction; reduces or avoids polymerization in the high pressure separator stage, and reduces or avoids polymerization of the monomer during recycle. It is furthermore amongst the objects to reduce and minimise suppression of the polymerization reaction resulting from carry over of the killer components in the recycled feed to the reactor. It is also one of the possible objects to provide a killer system which can be adapted easily to varying process requirements for producing different polymers of different molecular weights as well as different comonomer types and content at different polymerisation and elevated pressure separation conditions whilst making efficient use of the catalyst system.